Selective vacuum deposition of thin film



MTPOQ- Sept. 7, 1965 R. J. ALLEN SELECTIVE VACUUM DEPOSITION OF THIN FIT-Ill Filed Dec. 15, 1961 INV EV TOR. RICHARD J. ALLEN ii: fidwzl, M, j'

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United States Patent "ice 3,205,087 SELECTIVE VACUUM DEPOSITION OF THIN FILM Richard J. Allen, Baltimore, Md., assignor to Martin- Marietta Corporation, Baltimore, Md., a corporation of Maryland Filed Dec. 15, 1961, Ser. No. 159,565 1 Claim. (Cl. 117-237) This invention relates to a method and apparatus for vacuum deposition of thin films, and more particularly to means providing selective film deposition of variable patterns without requiring individual masks having the specific configuration of the pattern desired.

In the formation of printed circuits and the like, very thin metallic coatings have been formed through vapor deposition. In conventional systems, the metal to be deposited is generally heated in a crucible which is brought to the required temperature in a conventional manner such as by electric resistance heating. The vaporized metal is deposited on a base or substrate by the use of conventional masking techniques in which the areas not desirous of receiving the metallic coating are covered with a suitable mask having a specific configuration. In order to effect a change in the pattern, a mask having a different configuration must necessarily be substituted for the previously used mask.

While conventional methods of vapor deposition make use of resistance heating, it has been proposed to employ electronic rays for heating the crucible, or for directly heating the material to be applied to the substrate. In either case, the use of individual masks of specific configuration for each printed circuit pattern is still required.

It is, therefore, a principal object of this invention to provide an improved method of vapor deposition of a given image on a substrate in which the requirement of a specific mask for each different image is eliminated.

It is a further object of this invention to provide an improved method of vapor deposition of a material on a substrate in which a plurality of selective deposits may be formed simultaneously on single or multiple substrates from a single source.

It is a further object of this invention to provide an improved method of selectively vapor depositing a material on a substrate in which the deposit image may be varied without requiring any mechanical movement of the elements forming the deposition apparatus.

It is a further object of this invention to provide an improved method of selective vapor deposition for materials on a substrate in which high resolution of a deposit is achieved and in which deposit uniformity is enhanced.

Other objects and advantages of the invention will become apparent from the following detailed description, taken in connection with the accompanying drawing, in which:

FIGURE 1 is a simplified schematic elevational view, partly in section, of the apparatus used in performing the selective vacuum deposition process of the present invention.

Briefly, the method and apparatus of the present invention allows vacuum deposition of a material on variably selected portions of a substrate without requiring the need for a plurality of masks of specific configuration. The apparatus includes a beam source of electrons which are directed towards a mass of material to be deposited upon the substrate. The high energy electron beam contacting a surface of the material results in a spot source of vaporized material providing hemispheric radiation of the vaporized material. An orifice plate including a plurality of orifices is positioned in the path of the vaporizable material with a substrate positioned on the side of the orifice plate opposite the spot source. The

3,205,087 Patented Sept. 7, 1965 angle of incidence of the vaporizable material causes a.

reflected image of the material to be deposited on a portion of the substrate surface. Means are provided for deflecting the electron beam to cause the angle incidence to vary, thus varying the area of deposit of the material on the surface of the substrate. Electrostatic or electromagnetic means may be employed for deflecting the electron beam, these means being positioned intermediate of the electron beam source and the vaporizable material. In one embodiment of the invention, means are provided for selectively positioning a number of vaporizable materials in the path of the electron beam to effect multiple deposits of material on the substrate surface. Programming means may be employed for selectively deflecting the electron beam and movement of the material support to achieve predetermined image patterns on the substrate surface.

Referring now to the drawing, there is shown in one embodiment the apparatus employing the method of the present invention. The apparatus includes a suitable vacuum chamber including a belljar 8 positioned upon a base member 10 through which protrudes a shaft 12 rotatable by means 13 and having a horizontal support member 14 attached thereto. Member 14 serves to support a number of rectangular elements of metal such as 16 and 16' forming the material to be vacuum deposited. Suitable sealing means 18 surrounds the shaft 12 to insure retention of the vacuum within the apparatus. The system makes use of an electron gun 20 which may take the form of a cathode ray tube of conventional construction and acts to provide a source of electrons in beam form 22 which are directed towards the surface of the material 16 to be vapor deposited. The electron gun is focused on a small portion of the flat surface 24 of material 16, hereinafter referred to as the source. The area on which the beam is focused, hereinafter called the spot 26 is as small as practical. Existing equipment can provide spo diameters of .001 to .040 inch which are satisfactory for this invention.

An orifice plate 28 containing a plurality of small orifices 30 is mounted at some distance above the source spot 26 but in a position so as not to interfere with the electron beam 22. In the form shown, the

orifice plate 28 takes the form of a truncated cone having a hollow central portion 32 allowing the passage of the electron beam 22 therethrough. On the side of the orifice plate 28 opposite that from the spot source 26, there is provided a plurality of substrates 34 which are positioned on suitable support means 36 at a distance i, from orifice plate 28. Substrates 34 are positioned in a like manner to the orifice plate 28 so as not to interfere with the electron beam 22. The electron gun 20 directs the electron beam 22 at the source 16. Spot 26 formed on the surface may be moved for instance, by varying the potential across suitable deflection plates 38 and 40 as is conventional in cathode ray tube operations. In the place of electrostatic deflection, magnetic deflection may be used if desired. Energy imparted to the electrons through their acceleration, is converted into heat upon striking the source 16, causing it to melt and vaporize in the spot area 26. This provides a vapor source which approaches the ideal point source for a single hemisphere and which may be moved at will by varying the deflection volt-age.

The orifice 30 in the orifice plate acts as a pinhole in a pinhole camera, restricting passage of all but a narrow beam of vaporized material. As the spot 26 is moved, the beam 42 of vaporized material passing through orifice 30 sprays a deposit on the substrate which is a reversed image of the spot location 26, modified in size by the factor f /f the ratio of the distances of the spot 26 and substrate 34 to the orifice plane 28.

Since the spot radiates vapors uniformly over most of the hemisphere, any number of orifices 30 and substrates 34 may be employed, compatible with available space. In the apparatus shown, a number of sources 16 and 16' are positioned radially upon the horizontal support member 14 which may be in the form of a disc. Rotation of shaft 12 causes the individual sources 16, 16 to be successively positioned in the path of the electron beam 22 to permit successive deposits of different materials upon the substrate in a pattern determined by the deflection of beam 22 as a result of change in potential of the deflecting plates 38 and 40. The deflection of the electron beam 22 may be programmed, by means of a computer (not shown) making use of magnetic tape, punch cards, or other storage means for storing a great number of formats giving desired circuit configurations or portions thereof. Thus. automatic control circuit fabrication can be achieved in which both the configuration of the pattern form and the particular material to be deposited may be preselected completely by programming means.

The electron beam is much easier to work with than trying to deflect the higher mass molecular or vapor beam to change the angle of incidence of the vapor beam 22 passing through orifice 30. At the same time, the heavy mass of the vaporized molecules forming beam 42 acts to reduce the diffraction of the beam through the orifice.

The apparatus of the present invention provides for high resolution allowing the formation of an extremely thin deposit line on the substrate. In the specific apparatus, an electron gun employing voltages of 15 kv. to 20 kv. was found to be satisfactory. There is a possibility of some X-ray propagation; a base plate and a leadglass belljar are effective in stopping radiation. To prevent radiation from penetrating through a conventional O ring used to seal the jar to the plate, a lead baffle may be used. The existing electron guns operate at to kv. and 100 milliamps giving a maximum power of 2 kw. Since this may be focused on a one-sixteenth inch diameter spot, the resulting power density is .655 megawatt per square inch. Using lead as the source for the present example, it would require 9,000 joules per gram to vaporize the lead. A one-sixteenth inch diameter spot of lead, one millimeter deep, weighs approximately 0.0235 gram and would require 21.2 joules for vaporization. With a power density in the area of 0.655 megawatt (2000 joules/sec- 0nd), the time required to evaporate the spot is 0.01 second. If the source is considered an isotropic radiator in one hemisphere, and the orifice plane 28 is located at a distance of 10 centimeters, the source radiates into a surface area of 628 square centimeters. If the orifice is considered to be 0.01 centimeters square (an area 10 centimeters) the ratio of material passing through the hole to total material radiated is 0.l6 -10 Assuming this geometry results in a deposit area of twice the orifice area, the whole thickness is 80 Angstroms per millimeter of source depth evaporated. This gives a reasonable value for very thin film work. Reducing the spacing to five centimeters gives a 320 Angstroms per millimeter ratio, suitable for cryogenic circuitry.

From the above, it is evident that the efficiency of metal deposition is relatively low. However, if the efficiency in the case of all masked vacuum deposition is very low. Since the material cost is quite low and the desired mass of deposit is also quite low, it permits a low etficiency to be practical. An accurate comparison of the efiiciency for the proposed technique with professiona masking techniques may be had by comparing the ratio of mask-line area or orifice area. In the case of repetitive circuitry, the method and apparatus of the present invention is particularly advantageous since many orifices 30 are placed in the orifice plane 28 at one time creating many components for circuits simultaneously. In the drawing, it is noted that a great number of orifices 30 are provided each associated with an individual substrate 34 positioned behind the orifice plate. Individual substrates 34 may be replaced by a single substrate covering the complete area of the orifice plate 28 or a lesser number of substrates 34 of somewhat greater area, as desired. The overall efficiency of the system is of course improved by an extent equal to the number of orifice supplied.

It is apparent from the preceding description, that virtually all parameters may be varied to meet specific application needs. "For instance, the spot diameter, the path length, the orifice size and location, the substrate size and location may be varied with relative ease. In a highly repetitive circuit, such as a matrix, the orifice density could run in the order of many thousands per square inch. Since it is possible that some ionization may occur due to the presence of the electron beam 22, it may be necessary to apply a potential to the orifice plate to insure passage of the vaporized material through the orifice plate. At the same time, the thickness of the plate surrounding the orifice should be kept small relative to orifice diameter to minimize edge effects for small angles of incidence of the impinging vapor stream. It is envisioned by the present invention that the method and apparatus of the present invention may be advantageously applied to any microminiaturized circuitry, and in particular to the field of cryogenic electronics and the broad field of grown solid-state circuitry.

While there have been shown, described and pointed out the fundamental features of the invention as applied to a preferred embodiment, it will be understood that various additions and substitutions and changes in the form and detail of the device illustrated and in its method of operation may be made by those skilled in the art without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the following claim.

What is claimed is:

A method of depositing vaporizable material on selective portions of a substrate within a vacuum chamber without requiring a mask of specific configuration comprising; supporting a vaporizable material within said vacuum chamber, directing an electron beam on the surface of said vaporizable material to form a spot source of vaporized material, placing an orifice plate including at least one orifice in operative position with respect to said spot source, placing a substrate over said orifice on the side of said plate opposite said spot source, and deflecting said electron beam to cause said spot to move on said material whereby said area of deposit on said substrate is selectively varied in accordance therewith.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCES Strong, Procedures in Experimental Physics, Prentice- Hall, NY. (1938), pp. 172 to 174 relied on.

RICHARD D. NEVIUS, Primary Examiner. 

